1. Field of the Invention
This invention relates to the calibration of a manipulator. More particularly, it relates to the precise calibration of an articulated robot using a precision calibration fixture and then compensating for errors in the articulated robot.
2. Description of the Prior Art
In the prior art, one known method is to align by sight marks scribed on the arm with marks on clear plastic lenses attached to the arm. This provides a known angle for each of the joints for each scribe mark. There are a series of scribe marks at regular intervals for each joint. More than one set of marks is required when a measuring device such as a potentiometer, nonlinear in nature, is used.
The obvious problem with this method is the difficulty in aligning the two marks with each other. Another problem is accurately placing the marks on the arm. The width of the scribe marks is a limiting factor because it puts a limit on the resolution to which the angles can be measured.
Another calibration method is one wherein a bubble sight level is used for determining a set of known angles by placing each segment in either a horizontal or a vertical position or at a known angle by using a protractor in conjunction with the bubble sight level. The segments are assumed parallel to the axis between joint centers.
Disadvantages of this method are the need for true surfaces on the exterior of the segments, difficulty in placing the segments in a desired position and inaccuracies in reading the bubble level.
Still another prior art scheme uses a combination of a potentiameter and digital encoder to measure each joint angle. A potentiameter is used to adjust the offset between measured and true zero, while the encoder measures incremental changes in position. In most cases, the digital encoder is sufficiently linear so that no adjustments for a non-linearity must be made.
The calibration procedure requires that the robot arm be moved to an initial vertical position. Grooves or marks on the arm are used to align the outer joints. The other joints are positioned in vertical position either by sight or by the use of a plumb bob or a level-protractor. Once the arm is in position, the arm power is turned off and potentiometers are adjusted to make the joint angles read their expected values. The system's software then calculates encoder scale factor, encoder offset, potentiometer scale factor and potentiometer offset. Calibration data is ordinarily lost if power is shut off.